Phosphorus deficiency inhibits growth in parallel with photosynthesis in a C3 (Panicum laxum) but not two C4 (P. coloratum and Cenchrus ciliaris) grasses

This study compared the growth and photosynthetic responses of one C₃ (Panicum laxum L.) and two C₄ grasses (Panicum coloratum L. and Cenchrus ciliaris L.) to changes in soil phosphorus (P) nutrition. Plants were grown in potted soil amended with six different concentrations of P. One week before harvest, leaf elongation and photosynthetic rates and the contents of carbohydrate, P and inorganic phosphate (P_i) were measured. Five weeks after germination, plants were harvested to estimate biomass accumulation. At each soil P supply, leaf P contents were lower in the C₃ (0.6-2.6 mmol P m^-2) than in the two C₄ grasses (0.8-4.1 mmol P m^-2), and P_i constituted ~40-65% of total leaf P. The P deficiency reduced leaf growth, tillering and plant dry mass to a similar extent in all three grasses. In contrast, P deficiency suppressed photosynthetic rates to a greater extent in the C₃ (50%) than the C₄ grasses (25%). The foliar contents of non-structural carbohydrates were affected only slightly by soil P supply in all three species. Leaf mass per area decreased at low P in the two C₄ grasses only, and biomass partitioning changed little with soil P supply. The percentage changes in assimilation rates and plant dry mass were linearly related in the C₃ but not the C₄ plants. Thus, P deficiency reduced growth in parallel with reductions of photosynthesis in the C₃ grass, and independently of photosynthesis in the two C₄ grasses. We propose that this may be related to a greater P_i requirement of C₄ relative to C₃ photosynthesis. Photosynthetic P use efficiency was greater and increased more with P deficiency in the C₄ relative to the C₃ species. The opposite was observed for whole-plant P-use efficiency. Hence, the greater P-use efficiency of C₄ photosynthesis was not transferred to the whole-plant level, mainly as a result of the larger and constant leaf P fraction in the two C₄ grasses.